The present invention relates to a toner density sensing apparatus for an electrostatic copying machine or the like.
A toner or developing substance for a dry process electrostatic copying machine comprises ferromagnetic carrier particles and non-magnetic, black colored toner particles. The toner particles adhere to the carrier particles due to frictional electrostatic force. The toner mixture is applied to a photoconductive drum on which an electrostatic image of an original document is formed by means of a magnetic brush which adheres the toner mixture thereto by means of magnetic attraction of the carrier particles.
The toner particles are attracted from the magnetic brush to areas of high electrostatic charge on the drum to develop the electrostatic image into a toner image which is transferred and fixed to a copy sheet to provide a permanent reproduction of the original document corresponding to the electrostatic image.
Whereas the toner component is progressively consumed in the developing process, the carrier component is not. Thus, the toner density, or the ratio of the toner component to the carrier component in the toner mixture, progressively decreases during the developing process. It is necessary to periodically replace the consumed toner particles and maintain the toner density substantially constant. Excessive toner density causes gray background areas on finished copies. Low toner density results in copies of insufficient image density. The acceptable range of toner density is quite narrow. For this reason, it is necessary to provide accurate means for measuring or sensing the toner density and replenishing the toner component when necessary.
A known method of sensing toner density involves causing the toner mixture to fall through a vertical conduit. An electromagnetic coil is wound around the conduit and energized with an alternating electric signal. The toner mixture in the conduit effectively constitutes a core of the coil. The permeability of the toner mixture and the inductance of the coil increase as the toner density decreases. The inductance reactance of the coil to the alternating electric signal thereby constitutes a measure of the toner density. The coil may be connected in the tank circuit of an oscillator in such a manner that the frequency of the oscillator varies in accordance with the inductance of the coil and thereby the toner density.
Although this type of toner density sensing apparatus is effective in theory, it does not provide satisfactory performance in actual practice. The reason for this is that the variation of inductance of the coil for a given change in toner density is quite small. Measuring apparatus with sufficient sensitivity to detect such small changes in inductance is complicated in construction and expensive to manufacture. Even where such sensitive measuring apparatus is provided, the accuracy of toner density sensing is unreliable.